This invention relates generally to apparatus for removing contaminants from gaseous streams, and more specifically relates to venturi scrubbers.
One well known type of device for removing contaminants from a gaseous effluent stream (such as flue gas) is a venturi scrubber. In such device the effluent gas is flowed through a venturi tube having a narrow throat portion. As the gas moves through the throat it is accelerated to a high velocity. A scrubbing liquid or slurry is added to the venturi, usually at the throat, and enters the gas flow. The scrubbing droplets used are generally much larger than the contaminant particles or contaminant gases to be collected and, as a consequence, accelerate at a different rate through the venturi. The differential acceleration causes interactions between the scrubbing droplets and the contaminants, such that the contaminant particles or gases are collected by the droplets. The scrubbing droplets are then removed from the effluent stream which is thereby cleansed.
The inventors"" experience in operations at typical power plants indicates that severe gas maldistribution can exist in certain types of venturi scrubbers and that this contributes to a number of serious equipment problems. The symptoms are slurry carryover and buildup in the scrubber mist eliminators, induced draft fans, duct work, and second stage venturi absorbers that may be currently only used for mist elimination. In such systems liquid can even escape a second venturi mist eliminator and be re-entrained into the flue gas exiting the chimney, resulting in liquid droplets falling from the plume (termed xe2x80x9cstack rainxe2x80x9d).
The reasons for the above difficulties can be understood by reference to FIG. 1 which is a schematic block diagram depicting a prior art venturi scrubber flow system for sulfur dioxide and fly ash removal to which the present invention is applicable. FIG. 1 may be considered simultaneously with the prior art showing of FIG. 2 which depicts the venturi scrubber vessel in greater detail. The system 10 shown in FIG. 1 includes a scrubbing vessel 12 which is of a venturi-type known in the prior art. Exhaust flue gas 14 from a furnace 16 is provided to the inlet 18 of vessel 12. After being contacted with a slurry reagent which is sprayed via spray head 20 at the top of vessel 12 and by spray head 22 at an intermediate point, the flow of flue gas and spray droplets proceeds about a plumb bob body 24 and is then converged and enters the constricted venturi passage 26 where increased contact between the scrubbing slurry and flue gas is enabled. The slurry reagent 28, as for example an aqueous slurry of lime or a limestone, is fed into a slurry reservoir 30 at the bottom of vessel 12. The slurry level 32 in reservoir 30 is maintained at a relatively constant point within the bottom of the vessel. The slurry having contacted the flue gas descends to this underlying reservoir 30 where it is collected. The collected slurry is refreshed by reagent 28 and makeup water is added as neededxe2x80x94e.g., from wash water used for mist eliminator 50. The slurry is recirculated via the recirculation pump 34 to the spray heads 20 and 22. The solids are recovered from the slurry via the output line 36 which proceeds to a conventional thickener 38. A valve 35 opens to admit slurry from the reservoir to thickener 38 upon the solids in the reservoir exceeding apreset point. (Controls for valve 35 are not shown.) The thickener underflow proceeds via line 40 to a disposal or collection point.
It is seen that the vessel 12 is provided with an external wall 42, and an internal wall 44 which converges in the direction of gas stream flow and then defines a boundary for the constricted venturi passage 26 by its downwardly extending portion 46. Portion 46 terminates well above the slurry level 32. Thus it is seen that the flow of the gases and entrained slurry to be scrubbed, as depicted by the large arrows 53 in FIG. 2, proceeds downwardly through venturi passage 26 and then is turned about through approximately 180xc2x0 beneath the wall 46 and above the slurry level 32, and enters the annular space 48 defined between the aforementioned internal and external walls 46 and 42. The gases then proceed upwardly in annular space 48 and impact and pass through a conventional mist eliminator 50. An outlet duct 52 intersects annular space 48 at one side thereof (at the right in the sense of the drawing) and allows the gases proceeding above the mist eliminator 50 to exit and proceed via line 54 to stack 56 for discharge.
The two curves designated xe2x80x9ccurrent operationxe2x80x9d in FIG. 5 depict a typical flow velocity distribution in a prior art venturi scrubber vessel of the type shown in FIGS. 1 and 2, and illustrates the gas maldistribution problem occurring for the velocities near the exterior walls of the annular space 48. The dotted curve represents results for a computerized fluid dynamics (xe2x80x9cCFDxe2x80x9d) model for a plane 1 foot below mist eliminator 50. The curve defined by triangles depicts measured values at a plane about 5 feet below the mist eliminator 50. The parallel lines in the middle of the graph represent the downwardly extending input channel, i.e., the venturi passage 26. As seen from this graph, the flow velocity of the mist-entraining gases is vastly different in the annular space 48 depending upon whether one is considering the upward velocity of the gases adjacent internal wall 46 or the velocity as one approaches the external wall 42 of the vessel. In fact it will be noted that adjacent wall 46 the gases actually have a negative velocity indicative of eddying and swirling. Even though in the design considered the average velocity is only of the order 10 fps, the regions of high velocity typically exceed the 15 fps design velocity of the mist eliminator used in the present system, which results in slurry penetration through the mist eliminator and into downstream equipment. It is this marked nonuniformity in velocities in the annular space 48 which the present invention is intended to remedy.
In accordance with the present invention, a venturi scrubber is provided for scrubbing a gas stream to remove undesired gaseous components, which includes a scrubber vessel having an external wall, an upper inlet for admitting the gas stream, and a reservoir at the bottom of the vessel for collecting the scrubbing liquid sprayed into the vessel. An internal wall extends from the upper part of the external wall to define an axial converging passage for converging the gas stream from the inlet, and an adjoined downwardly extending restricted venturi passage receives the downwardly flowing gas stream from the converging passage. The internal wall abounding the venturi passage terminates above the liquid level at the reservoir. An annular space is thereby defined between the internal and external walls. An outlet duct for the scrubbed gas stream intersects the annular space at its upper reaches. The gas stream flowing downwardly through the converging passage and venturi passage is turned about the bottom of said internal wall to enter the annular space and proceeds therein upwardly and exits the vessel through the outlet duct. Spray means are provided for spraying a scrubbing liquid or slurry into the gas stream, and an annular mist eliminator is mounted in the annular space at an axial point below the outlet duct.
Pursuant to a first aspect of the invention a gas diverter means is mounted in the annular space below the mist eliminator to divert the gas stream which is turned upwardly into the annular space away from the external wall and toward the axis of the vessel, whereby the velocity of the upward flow through the annular space to the mist eliminator is rendered more uniform. The gas diverter is preferably an annular ring the transverse cross-section of which is adjacent to but slightly spaced from the external wall, and which is tilted away from the wall as to be convergent in the upward direction.
In a further aspect of the invention an outlet compensating means can be mounted in at least the sector of the annular space underlying the outlet duct, for impeding the flow of the gas stream toward the outlet and thereby compensating for the otherwise higher gas stream flow in such sector than in azimuthally displaced sectors of the annular space thereby rendering the upward stream flow in the annular space more uniform. The outlet compensating means can comprise a plate mounted above the mist eliminator and below the outlet duct, the plate having a plurality of portions closed to gas stream flow in the sector underlying the duct intersection and the plate being increasingly open to stream flow in its sectors which are azimuthally displaced from the duct intersection.